Conventionally, an image forming apparatus employing the electrophotographic method, such as a copier, printer, and facsimile, has been known. The image forming apparatus employing the electrophotographic method forms an electrostatic latent image on the surface of a photosensitive drum (toner image carrier), causes a developing device to supply toner to the photosensitive drum to develop the electrostatic latent image, transfers a toner image formed by development on the photosensitive drum to a sheet, such as a recording paper, and causes a fixing device to fix the toner image to the sheet.
The image forming apparatus has recently been adapted to demands for full color images and high quality images, and a two-component developer having superior toner electrifying stability is often used in such an image forming apparatus. This two-component developer is composed of toner and a carrier. The toner and carrier are rubbed against another when they are stirred in the developing device and properly charged toner is obtained due to the friction.
In the developing device, charged toner is supplied, for example, to the surface of a developing roller. Toner supplied to the developing roller is moved to an electrostatic latent image formed on the photosensitive drum by an electrostatic attraction force. As a result, a toner image based on the electrostatic latent image is formed on the photosensitive drum.
The image forming apparatus which is faster in processing speed and smaller in size has been demanded in recent years. This brings a need of quick and sufficient electrification of the two-component developer and faster transport of the two-component developer.
Under such circumstances, a circulatory developing device disclosed in Japanese Laid-Open Patent Publication No. 10-63081 has been adopted to cause replenished toner to disperse instantly into the two-component developer to give toner proper amount of electricity in the image forming apparatus. The circulatory developing device has a two-component developer transport passage through which the two-component developer is circulated and a two-component developer transporting member that transports the two-component developer while stirring it through the two-component developer transport passage.
The above circulatory developing device offers an advantage of miniaturizing the developing device, but also accompanies a disadvantage that a transport roller must be rotated at high speed because it takes a longer time for replenished toner to reach a development area when a large amount of toner is consumed. High-speed rotation of the transport roller, however, causes the stagnation of the developer, which is a phenomenon that the developer gathers into a loosely aggregated state in a gap between the inner wall of the developing device and the transport roller to become slow in movement.
This leads to a problem that when a permeability sensor disposed on the inner wall of the developing device detects the toner concentration of the two-component developer, a bad flow of the developer on the surface (detection surface) of the permeability sensor destabilizes output of the sensor.